Digitally controlled rectifying system for driving a motor

ABSTRACT

A digitally controlled rectifying system used for driving a d.c. motor through antiparallel connected forward and reverse converters. The firing phase angle of the converters is shifted by 180° at a time of gate switching operation for the converters so that the load current is formed to become zero, and after the gate switching operation the firing phase angle is shifted to a position at which the load current is substantially zero, whereby the time lag of control occurring at the switching operation of the converters can be minimized irrespective of the load condition.

This application is a continuation-in-part of copending application Ser.No. 616,475, filed June 1, 1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digitally controlled rectifyingsystem used for driving an electric motor.

2. Description of the Prior Art

Conventionally, there have been used rectifying systems of the analogcontrol type for driving electric motors as shown in FIG. 1. The systemarrangement of FIG. 1 includes a motor driving rectifying system 1, ad.c. motor 2, a forward converter 3 made up of a 3-phase thyristorbridge, and a reverse converter 4 also made up of a 3-phase thyristorbridge. The control system is of a feedback system comprising a speedcontrol loop and two minor loops of the current and voltage feedback.

The rectifying system 1 incorporates a speed controller 5 made of anoperational amplifier configured to provide proportional and integralfunctions, a terminal 6 for receiving a speed command signal, a terminal7 for receiving a feedback speed signal which is produceduninterruptedly by a pilot generator 8 associated with the motor 2, acurrent controller 9 made up of an operational amplifier configured toprovide an integral function, an output terminal 10 of the speedcontroller 5 which provides the current command signal, a terminal 11for receiving a feedback current signal which is produced in response tothe input a.c. current detected by an a.c. current transformer (ACCT)12, a voltage controller 13 made up of an operational amplifierconfigured to provide a first-order time lag function, an outputterminal 14 of the current controller 9 which provides a voltage commandsignal, a terminal 15 for receiving a feedback voltage signal which isproduced in response to the d.c. output voltage by a voltage sensor 16provided across the motor 2, a gate pulse generator 17 incorporating acos⁻¹ function for providing a linearized output voltage for a phasecommand signal with a bias voltage Eb being applied thereto in order tostabilize the gate switching operation for the forward and reverseconverters 3 and 4 as will be explained later, a forward/reverseswitching logic circuit 18 for selecting the output of the gate pulsesignal (a) and (b) for the forward and reverse converters 3 and 4, andgate pulse switches 28a and 28b for conducting gate pulses to theforward and reverse converters 3 and 4.

The forward/reverse switching operation of the foregoing conventionalrectifying system will be described.

Symbols ⊕ and ⊖ shown at the inputs and outputs of the speed, currentand voltage controllers 5, 9 and 13 represent the polarity of respectivecontrol signals when the motor 2 rotates in a forward direction.

FIG. 2 is a graph used to explain the correlation between the inputsignal of the gate pulse generator 17, i.e., the output signal 19 of thevoltage controller 13, and the d.c. output voltage. In the figure, thesolid line shown by A represents the characteristics when the loadcurrent flows continuously, the shaded portion shown by B represents thecharacteristics when the load current flows intermittently, and thedashed lines shown by C represent the state in which the load current iscompletely cut off. The provision of the dead band is to prevent theforward and reverse converters 3 and 4 from becoming conductivesimultaneously, and for this purpose the output characteristics of bothconverters are biased by a predetermined amount of ±Eb volts withrespect to the output signal of the voltage controller 13. The voltagelevel shown by Ec in the graph represents the counter electromotiveforce produced by the motor 2 when it rotates in the forward direction.When the speed controller 5 is given a deceleration command via theterminal 6, its output signal at the terminal 10 decreases from apositive value and then enters the negative region. Since the currentcontroller 9 has an integral property, its negative output signal at theterminal 14 when evaluated as an absolute value starts decreasing at arate proportional to the product of the reciprocal of the integratingtime constant and the input error signal. On the other hand when theoutput signal 19 of the voltage controller 13 has decreased down to theinput signal level V₁, the load current becomes a complete zero, causingthe output signal of the current controller 9 to vary solely in responseto the current command signal at the terminal 10.

When the voltage controller 13 has reversed the polarity of the outputsignal from positive to negative, the switching logic circuit 18operates on the gate pulse signal by its output signals Sa and Sb toswitch from the forward converter 3 to the reverse converter 4. When theoutput signal 19 has reached the input signal level -V₂, the currentstarts flowing through the reverse converter 4, bringing the motor 2 ina regenerative operating mode, and the motor speed starts falling. Inthe meantime when the output signal 19 varies from the input level V₁ to-V₂, no load current flows in the motor 2, and it is an idle time forthe control system. This idle time cannot be nullified due to the biasEb provided for the output of the voltage controller 13 in order toprevent a short-circuit of the forward and reverse converters 3 and 4,and to make the matter worse the idle time is significantly affected bythe gain of the current controller 9 and voltage controller 13.

Therefore, it has been logically impossible for the conventional motordriving rectifying system of the analog control type to reduce theswitching time because of the need for the bias voltage Eb forstabilizing the switching operation of the forward and reverseconverters. Moreover, the gain of the current controller 9, which isdetermined depending on the properties of the motor and load, couldcause a very long idle time for the converter switching operation insome cases.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a digitallycontrolled motor driving rectifying system which overcomes the foregoingprior art deficiencies.

Another object of the present invention is to provide a high responseand high controllability rectifying system which considerably reducesthe gate switching time and, thus, eliminates the time lag of thecontrol system.

Other objects and advantages of the present invention will be apparentfrom the following detailed description of a certain preferredembodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the conventional motor drivingrectifying system of the analog control type;

FIG. 2 is a graph used to explain the relationship between the inputsignal of the gate pulse generator and the d.c. output voltage in theoperation of the system shown in FIG. 1;

FIG. 3 is a block diagram showing the motor driving rectifying system ofthe digital control type embodying the present invention;

FIG. 4 is a detailed circuit diagram of the high speed switching circuitof FIG. 3;

FIG. 5 is a flow chart showing the operation of the high speed switchingcircuit of FIG. 4;

FIG. 6 is a waveform diagram showing the relationship between therectifier output voltage and the counter electromotive force of themotor during the continuous current operating mode of the system shownin FIG. 3; and

FIG. 7 is a waveform diagram showing the relationship between therectifier output voltage and the counter electromotive force of themotor during the zero-current operating mode of the system shown in FIG.3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will now be described in detailwith reference to the drawings. FIG. 3 shows in block form themicroprocessor based motor driving rectifying system embodying thepresent invention. In the figure, the same portions or equivalentportions to those of FIG. 1 are given the common reference numbers, andexplanation thereof will be omitted. In FIG. 3, a section 20 defined bythe dot-and-dash line is a motor driving rectifying system, and a minorsection 30 defined by the dots-and-dash line is a microprocessor basedcontroller. Reference number 23 denotes a motor speed measuring unitwhich counts pulses from a pulse generator 22 coupled to the drive shaftof the motor 2 and provides a digital value in correspondence to therotational speed of the motor 2. The controller 30 has a speed commandinput terminal 21, a block 24 which corresponds to the speed controller5 of FIG. 1, and another block 25 which corresponds to the currentcontroller 9 of FIG. 1. Both the speed controller 24 and currentcontroller 25 perform computation for the proportional and integralcontrol. The output of the current controller 25 is conducted through ahigh-speed switching logic circuit 27, as will be described later, to agate pulse generator 26 which is constituted by discrete digitalcomponents. The gate pulse generator 26 incorporates a cos⁻¹ functiongenerator. The gate pulse generator 26 differs from the pulse generator17 (FIG. 1) in the conventional system in that the input is not biased.

The high-speed switching logic circuit 27, which is the most materialportion of this embodiment, will be described in connection with FIGS. 4and 5.

FIG. 4 is a circuit diagram showing the details of high speed gateswitching logic portion 27 of an electric motor driving rectifier shownin FIG. 3, and FIG. 5 is the flowchart thereof.

The condition to determine whether the gate switching operation isrequired or not is judged from the polarity of the reference currentsignal, that is, the output from a speed controller 24. (S₁ in FIG. 5)Namely, if the reference current signal is positive in its polarity, thepositive side (FWD) gate must be ignited. If the present gate is ignitedin reverse side (REV), the process should be advanced to gate switchinglogic portion 27 so as to make gate switching operation.

When the reference current signal is not reversed in its polarity, thegate switching logic portion 27 is neglected.

Even if the reference current signal is negative in its polarity, theprocess should be carried out in the similar way to advance to theswitching logic portion 27 when the present polarity is positive and toneglect the logic portion 27 when the polarity is staying unchanged.

The part ○A in FIG. 4 is the circuit corresponding to this step S₁, andthe output from a comparator 41 on positive side is input in D-flip-flop43 when the input signal into the comparator 41 changes from thenegative value to a positive value. On the other hand, if the inputsignal into the comparator 42 changes from the positive value to anegative value, the output from the comparator 42 on the reverse side isinput in D-flip-flop flop 44 whose output changes to H level.

Next, when the reference current signal is reversed, the intermittentON/OFF state can take place immediately provided that the current loopis making normal operation. This condition should be confirmed. (S₂ inFIG. 5)

The circuit part corresponding to this step is shown by ○B in FIG. 4,and when intermittent current ON/OFF detector 45 detects anyintermittent current, flip-flop 46 or 47 is set and either output goesup to "H" level.

Furthermore, if the state of intermittent current is detected, the gatephase α is shifted to 180° until the current is perfectly reduced tozero. (S₃ in FIG. 5)

The circuit corresponding to this step S₃ is shown by ○C in FIG. 4.

Namely, when the output from either flip-flop 46 or 47 goes up to "H"level, the output from a current controller 25 is cut off from the gatecircuit and a bias corresponding to α=180° is input in the gate circuit.

When a zero current detector 48 detects the zero current under thiscondition, flip-flop 49 or 50 is set and either one of already setflip-flops among the flip-flops 43, 44, 46, 47 is reset simultaneously.

After the zero current is recognized, the output from the currentcontroller 25 (integral item of proportional integration) is set to thevalue corresponding to ##EQU1## so as to make gate switching operation.(S₄ in FIG. 5)

The circuit corresponding to this step S₄ is shown by ○D in FIG. 4, andwhen the output from flip-flop 49 or 50 is reversed, the output from thecurrent controller 25 is set, and the gate changover switch is switchedfrom the present state simultaneously, and thus the operation isfinished. FIG. 6 shows the waveform of the output voltage Ed of therectifying system 20 and the counter electromotive force Ec of the motor2 when the load current Id flows continuously. FIG. 7 shows the waveformof Ed, indicating that the load current Id becomes zero when the peakvalue of the output voltage Ed is equal to the counter electromotiveforce Ec. In this case, the relationship between the firing phase angleα and the counter electromotive force Ec of the motor is expressed bythe following equation. ##EQU2## where Es is the effective input linevoltage.

The high-speed gate switching logic circuit 27, when it does not performgate switching, passes the proportional and integral output from thecurrent controller 25 directly as a firing phase angle command signal tothe gate pulse generator 26. On the other hand, to carry out gateswitching the logic circuit 27 shifts the firing phase angle α by 180°so that the load current is forced to become zero. Upon detection of thezero load current Id, the logic circuit 27 reverses the states of thegate pulse switches 28a and 28b, and at the same time shifts the firingangle command signal so that the firing phase angle α meets the equation(1). The integral part of the output from the current controller 25 ismade equal to the firing angle command signal as obtained in the aboveprocess. The firing phase angle α at this time is expressed by thefollowing equation. ##EQU3## where Ec is the counter electromotive forceof the motor 2 detected by the voltage sensor 16 when the load currentId becomes zero. Since the firing phase angle α following the gateswitching operation has a critical value at which the load current iskept zero, the load current flows progressively as the firing phaseangle α advances.

According to the inventive motor driving rectifying system, as describedabove, the firing phase angle α is shifted by 180° when the gate controlof the forward and reverse converters is switched so that the loadcurrent is forced to become zero, and after the gate switching, thefiring phase angle is shifted to a phase angle at which the current juststarts flowing. These two properties enable a significant reduction ingate switching time as compared with the conventional system, whereby ahigh response and high controllability rectifying system without a timelag of control can be realized.

What is claimed is:
 1. A digitally controlled rectifying system used fordriving a direct current (d.c.) motor through a forward and reverseconverters connected in an anti-parallel fashion so as to block acirculation current, said system comprising:(a) a pulse generatorcoupled to the drive shaft of said motor; (b) a speed measuring meanswhich counts pulses generated by said pulse generator and produces adigital value in correspondence to the rotational speed of said motor;(c) a speed controller which receives a speed command signal and theoutput of said speed measuring means; (d) a current controller whichreceives the output of said speed controller and the output of a currenttransformer coupled to an alternating current (a.c.) input power line tosaid converters; (e) logic means which receives the output of saidcurrent transformer and the output of a voltage sensor connected todetect the output voltage of said converters, said logic means operatingon the firing phase angle of said converters to shift by 180° at a timeof gate switching operation so that the load current is forced to becomezero, said logic means operating on the firing phase angle of saidconverters to shift following the gate switching operation so that theload current is substantially zero; and switch means operated by saidlogic means to supply gate pulses to one of said forward and reverseconverters.
 2. A rectifying system according to claim 1, wherein saidspeed controller, said current controller and said logic means incombination comprise a microprocessor based controller.
 3. A rectifyingsystem according to claim 1 wherein sa said logic means comprises:(a) afirst means ( ○A in FIG. 4, S₁ in FIG. 5) for determining whether thegate switching operation is required or not is judged from a referencecurrent signal comprising said output of said speed controller andtransmitted from said speed controller to said first means; (b) a secondmeans ( ○B in FIG. 4, S₂ in FIG. 5) for effecting the intermittentON/OFF state immediately provided that the current loop is making normaloperation, (c) a third means ( ○C in FIG. 4, S₃ in FIG. 5) for shiftingthe gate phase angle α to 180° until the current is reduced to zero,when the state of intermittent current is detected, (d) a fourth means (○D in FIG. 4, S₄ in FIG. 5) for setting the output from said currentcontroller to the value corresponding to ##EQU4## α is the firing angleand Ec is the electromotive force so as to make gate switching operationafter the zero current is recognized.